The 'rule of thumb' calculation you are referring to is pretty linear when you chart it out, when, in fact, investing is not. Due to compound interest, your total investments grow much larger the longer you go out in the series (a.k.a. the older you get, assuming average returns are positive and you keep contributing the same or more each period), so I am like you. It is not a good calculation to go by for younger people, especially for the '30 year old' in the example.
[This message has been edited by gigemboy (edited 6/22/2013 5:40p).]